1. Field of the Invention
The present invention relates to frequency generation in video monitor. In particular, the present invention relates to frequency generation during switch-over for multi-frequency video monitor.
2. Description of Related Art
Multi-frequency video monitors operate over a range of frequencies to accommodate various display resolutions. Most personal computer (PC) video controllers can generate different synchronization signals for different display resolutions depending on the user's selection of display mode.
In video processing circuit, the frequency switching is performed by a number of electronic components, one of which is a horizontal output transistor. When a change in horizontal sync signal occurs, the horizontal output transistor has to respond quickly to generate the new horizontal sync signal to drive the horizontal control circuit. However, the frequent transitions from one frequency to another frequency may cause undesirable effects to the switching elements such as the horizontal output transistor in the defelection circuit. In some cases, the device can be damaged.
One simple way to prevent the damage is to slowly change the sync frequency from the old value to the new value so that the horizontal output transistor has sufficient time to respond. However, this method causes noticeable display artifacts and undesirable viewing.
Accordingly, there is a need to provide a method and apparatus for providing frequency generation during switch-over to avoid damage to the switching elements without noticeable display effects.